1. Field of the Invention
Selective and efficient multifunctional nanoporous catalysts and methods of preparation thereof.
2. Description of Related Art
Multi-step and efficient synergistic catalytic processes to various types of biomolecules by biological catalysts (enzymes) are very common in living organisms. Many notable examples of such enzymatic and antibody catalytic processes involve acid-base cooperative or efficient bifunctional catalysts. By mimicking these extraordinary abilities of nature, some conventional homogeneous bifunctional acid-base catalysts have been synthesized, such as those disclosed by Breslow et al. in J. Am. Chem. Soc. 1993, 115, 10988-10989. However, the efficiency and selectivity of these catalysts, which often depend on the relative separation distances between the acid and base catalytic sites, are often poor because the materials lack a continuous range of acidic and basic catalytic sites. Hence, a considerable amount of recent effort has been directed towards the synthesis of heterogeneous solid-state, acid-base catalysts having well-controlled, high concentrations of acidic and basic catalytic sites.
A family of mesoporous materials, which were first reported in 1992, has been widely and effectively used as hosts for a variety of catalytically active functional groups, including acidic and basic sites, to produce efficient heterogeneous catalysts. By postgrafting of the residual surface silanol groups of the mesoporous materials with organosilanes, high surface area and tunable nanopores containing solid-acid and solid-base mesoporous catalysts for reactions such as Knoevenagel condensation, catalytic oxidation, and Michael addition have been synthesized, as reported by Cauvel et al. in J. Org. Chem. 1997, 62, 749-751; and Rao et al. in Angew. Chem. Int. Ed. 1997, 36, 2661-2663. However, to the best of the applicants' knowledge, almost all postgrafting syntheses of catalysts reported to date are typically done by stirring mesoporous materials with excess organosilanes in non-polar solvents such as toluene or cyclohexane at reflux temperature, 112° C.
Postgrafting of organosilanes onto mesoporous materials in toluene in reflux indeed allows an effective inclusion of densely populated or high concentrations of covalently bound organic functional groups, including organoamines. However, this synthetic approach also has drawbacks as it grafts most of the surface silanol groups of the materials. The latter groups, which can act as weak acids, generally increase the efficiencies of a number of organoamine catalyzed reactions such as the Henry reaction and nitroaldol condensations. Furthermore, the presence of densely populated organic groups reduces the surface areas and pore volumes of the materials. Therefore, densely populated organoamine catalysts synthesized in toluene are typically accompanied by loss of catalytic efficiency. For instance, metallocene catalytic groups immobilized on densely populated postgrafted organoamine synthesized in toluene have lower catalytic efficiency for polymerization reactions than corresponding samples containing sparsely populated metallocene groups, as reported by McKittrick et al. in J. Am. Chem. Soc. 2004, 126, 3052-3053; and Hicks et al. in Chem. Mater. 2006, 18, 5022-5032. However, the synthesis of the latter materials involves a lengthy multi-step procedure consisting of preparation of bulky imine containing organosilanes and postgrafting the groups in toluene to form densely populated imine functionalized mesoporous materials. Upon subsequent hydrolysis of the bulky imine groups, spatially ordered organoamines and silanol groups are formed.
Recently, Katz et al. described the synthesis of organoamine functionalized silica gel catalysts containing silanol groups in J. Am. Chem. Soc. 2006, 128, 3737-3747. These bifunctional catalysts showed increased efficiency and selectivity for the Michael and Henry reactions compared to the corresponding materials without silanols. However, the surface area of silica gel is low, the number of the bifunctional groups in the material is limited, and the distribution of the two groups is difficult to control. Davis et al. have also reported the synthesis of sulphonic acid and organoamine bifunctionalized catalysts for nitroaldol reaction by self-assembly in Angew. Chem. Int. Ed. 2006, 45, 6332-6335. However, these materials have a low number of randomly distributed acid and base groups.
What is needed to address these problems is a catalyst material that can be synthesized by a simple, straightforward process, that has both acidic and basic functionality, and that has a high efficiency with respect to catalyzed reaction rate and yield.
Additionally, many pharmaceutical and industrial catalytic processes involve multiple, similar reactants and competitive reactions while a product from one of the reactants or reactions is only needed. The production of specific products by selectively catalyzing a specific reactant or reaction in a mixture of similarly reactive compounds or from competitive reactions is often necessary for the efficient production of various fine chemicals and industrial materials in high yields. Consequently, the development of selective catalysts and the efficient catalysis of one specific reactant have remained important research areas in catalysis and materials science; however, achieving these goals is often met with considerable challenges. For instance, by using the differences in the sizes and shapes of the reactants and their mass transport into the catalytic sites on solid zeolite porous supports, many selective catalysts have been synthesized. However, due to the narrow pore-sizes of zeolites, selective catalytic reactions of only smaller molecules are possible.
The recent advances in the synthesis of organic- and organometallic-functionalized mesoporous metal oxides and imprinted polymeric and imprinted metal oxide nanostructured materials have opened up synthetic strategies to novel selective catalysts. While the nanoporous structures in mesoporous materials enable size and shape selectivity as in zeolites, the higher pore diameters and the large surface areas in mesoporous materials further allow surface immobilization of large numbers of various organic groups to tune the surface properties and pore-diameters of the materials without severely clogging the pores as in zeolites. Functionalization of these materials with organic groups of specific hydrophobicity or hydrophilicity modifies the immediate dielectric environment of the catalytic site and enables reactant of matching polarity to access the catalytic sites and undergo preferential catalytic reactions. For instance, by co-condensation of two organosilanes, Lim and co-workers report in J. Am. Chem. Soc. 1997, 119, 4090-4091 the synthesis of bifunctional mesoporous organosilica materials that are selective to hydrophobic groups. These materials, however, achieved the required selectivity with a maximum of 50% yields in over 24 h and selectivity only for hydrophobic reactants. Co-condensation procedures in synthesis of mesoporous organosilica often results in poorly ordered mesostructures, which might be one of the reasons for the low yield of the Henry product by these materials. Very recently, Anwander and co-workers synthesized functionalized mesoporous silicas with two steps grafting in non-polar solvent, cyclohexane, to pore-size engineer cage like pores of SBA-15 materials for size-selective catalytic transformations, as disclosed in Chem. Eur. J. 2007, 13, 3169-3176. They used various sized long chain alkyl dimethylaminosilanes and organoaluminum compounds and they demonstrate aluminum-catalyzed Meerwein-Ponndorf-Verley reduction of differently sized aromatic aldehydes (benzaldehyde and 1-pyrenecarboxyaldehyde).
What is needed to address the problems where a single reaction pathway and product is desired from multiple pathways and products is a multifunctional catalyst that has tunable selectivity and that is simple and straightforward to synthesize.